[SalesForce] how to prevent the system generated “Customer Portal: Customer Portal Reset Password (SAMPLE)” Email Template

I think that it is not possible. There is an Idea to "Make New User Template Email Notification in Customer Portal more professional" http://success.salesforce.com/ideaView?id=087300000007XMqAAM.

The only problem is that the Idea is old so maybe it has been fixed and I just can't find out how (There is a 3 month old comment, though).

I'm 99% sure that there is no support for it. Salesforce did a half-you-know-what job with this (frustrated).

As you pointed out, the Receiving_User is actually a Contact object - this means there is no Username field on that Object. Depending on how you set up your users, I would assume that the Email field is actually used for generating usernames for customers. In this case, you can just change {!Receiving_User.Username} to {!Receiving_User.Email} and this would be merged with the user's username.

If that is not working, what you might do is use a visualforce template with an apex:component, supply the {!Receiving_User.Id} to the component's controller, query for the User with that ContactId and then return this User.Username to the VF-Email.